Phensuximide and methsuximide are two alpha phenlysuccinimides currently used in the treatment of epileptic seizures. They resemble ethosuximide in their effectiveness against pentylenetetrazol-induced seizures but differ in that they are also effective against maximal electric shock seizures in mice and rats. Both drugs have been marketed for the treatment of absence seizures for more than 20 years but they are infrequently employed, and rarely used alone. The purpose of this project is to determine in the two agents 1) the single dose plasma half-lives, 2) the relative plasma levels of the parent drug and its desmethyl metabolite with chronic administration, 3) the relationship of the efficacy of the parent drug and/or metabolite to the plasma levels, and 4) the efficacy against several seizure types. Patients admitted to the Clinical Center undergo a baseline week of observation, including a six hour video tape and telemetered EEG recording session. They are given single doses of phensuximide and then chronic doses of phensuximide which are later discontinued and then single doses of methsuximide and finally chronic methsuximide therapy. Patients who respond favorably to either of these drugs are encouraged to continue this medication.